Ion implanters are widely used in semiconductor manufacturing to selectively alter the conductivity of materials. In a typical ion implanter, ions generated from an ion source are transported downstream through a series of beamline components which may include one or more analyzer and/or collimator magnets and a plurality of electrodes. The analyzer magnets may be used to select desired ion species and filter out contaminant species or ions having undesirable energies. The collimator magnets may be used to manipulate the shape of the ion beam or otherwise adjust the quality of the ion beam before it reaches a target wafer. Suitably shaped electrodes can be used to modify the energy and the shape of the ion beam. After the ion beam has been transported through the series of beamline components, it may be directed into an end station to perform ion implantation.
FIG. 1 depicts a conventional ion implanter system 100. As is typical for most ion implanters, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a series of beamline components through which an ion beam 10 passes. The series of beamline components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a mass slit 112, a scanner 114, and a collimator magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 118.
As the semiconductor industry keeps reducing feature sizes of micro-electronic devices, ion beams with lower energies are desirable in order to achieve shallow dopant profiles for forming shallow p-n junctions. Meanwhile, it is also desirable to maintain a relatively high beam current in order to achieve a reasonable production throughput. Such low-energy, high-current ion beams may be difficult to transport within typical ion implanters due to space charge blow-up. To prevent “blow-up” of a positive ion beam, negatively charged particles, such as electrons or negative ions, may be introduced for charge neutralization. One way of sustaining space charge neutralization is through magnetic confinement of negatively charged particles. However, existing magnetic confinement approaches tend to introduce extra magnetic field components that cause ion beam distortion. Moreover, in order to improve low-energy beam transportation caused by space charge limitations, a high-energy ion beam may be decelerated to a desired energy level before reaching a target (e.g., a wafer). In such cases, some ions may go through “charge exchange” with surrounding neutral particles, thus losing their charge prior to deceleration and generating neutral particles having high energy. Neutral particles having high energy fail to be decelerated and may impact the target at a higher energy level than desired, thus negatively impacting implantation results.
Low-energy ion beams may be difficult to transport through the beamline to the target due to mutual repulsion between ions having the same charge. High-current ion beams typically include a high concentration of charged ions that tend to diverge due to mutual repulsion. To maintain low-energy, high-current ion beam quality, a plasma may be injected into the ion beam for the purpose of charge neutralization.
High-energy ion implantation beams typically propagate through a weak plasma that is a byproduct of beam interactions with residual or background gas. This plasma tends to neutralize the space charge caused by the ion beam, thereby largely eliminating transverse electric fields that would otherwise disperse the ion beam. However, for a low-energy ion beam, the probability of ionizing collisions with background gas is lower compared to a high-energy ion beam. In addition, low-energy ion beam blow-up may occur at much lower transverse electric field strength.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current techniques for transporting low-energy ion beams.